Photocatalytic conversion and kinetic study of CO₂ and CH₄ over nitrogen-doped titania nanotube arrays

The performance of highly ordered nitrogen-doped titania (TiO₂) nanotube arrays, fabricated by anodization method, was tested for photocatalytic CO₂ conversion with CH₄. Nitrogen-doped titania nanotube arrays were characterized using field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET), X-ray photoelectron spectroscopy (XPS) and Ultraviolet-visible (UV-vis) spectra. The photoreduction products were identified using residual gas analyzer (RGA) and GC spectra. The effects of important parameters such as UV light power, initial ratios of CO₂:CH₄:N₂ in feed and distance between UV lamp and reactor on CO₂ and CH₄ conversions were analyzed using response surface methodology (RSM). FESEM images of titania nanotube arrays indicated highly ordered and vertically oriented morphology with inside diameter ranging from 3 to 50 nm. The optimal conditions for maximum CO₂ conversion of 41.5% were determined as 250 W UV light power, 10% CO₂ initial ratio and 2 cm distance between UV lamp and reactor. H₂ and CO were the main products with selectivities being 80.5% and 18.9%, respectively. CO₂ and CH₄ molecules were competitively activated by the charge transfer excited complexes and the values of feed ratios influenced the selectivity for the formation of the desired products. The kinetic model based on Langmuir-Hinshelwood, incorporated photocatalytic adsorptive reduction and oxidation reactions over the catalyst surface fitted-well with the experimental data.